It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid developers comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored thermoplastic resin particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The said particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored thermoplastic resin particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound to the liquid developer comprising the thermoplastic resin, dispersant nonpolar liquid and generally a colorant. Such liquid developers, while developing good quality images, still do not provide the quality images required for certain end uses, e.g., optimum machine performance in digital color proofing. As a result much research effort has been expended in providing new type charge directors and/or charging adjuvants for electrostatic liquid developers. Higher quality image development of latent electrostatic images is still desired.
It has been found that the above disadvantages can be overcome and improved electrostatic liquid developers prepared containing an ionic or zwitterionic compound soluble in nonpolar liquid which have improved image quality or latent electrostatic images.